In the past decades, travel by aircraft has become commonplace, population has greatly increased particularly in urban and metropolitan areas, and the number of daily flights has expanded proportionately. Population density enlargement in the vicinity of airports in combination with a high frequency of takeoffs and landings has expectedly resulted in public criticism of nuisance, inconvenience, and damage created by noise inherent to the air travel industry. National and international agencies, aircraft manufacturers, engine manufacturers, and others have responded to the criticism by establishing noise emission standards for aircraft and aircraft traffic limitations for particular communities. The United States Federal Aviation Administration (FAA) has imposed noise limits on takeoff and landing.
Noise rules generally limit aircraft noise that can be emitted during takeoff and during approach to landing. Aircraft noise has many constituent parts including engine fan noise, engine combustion noise, airframe noise, and jet noise caused by shearing of airflow. Engine noise during takeoff is usually the largest noise component because the engine is then at the highest power setting. Jet noise is a prevalent engine noise component at high engine thrust conditions.
Many techniques for suppressing engine noise have been developed. In one example, engine secondary-to-primary mass flow bypass ratios are increased to values of five to eight to decrease peak jet velocities, shear layer velocity gradients, and turbulence, thereby reducing noise. Some aircraft use “hush kits” such as ejectors or free mixers, and forced mixers to mix high velocity hot engine streams with cooler low velocity freestream air to decrease peak jet velocity and shift from low frequency to more absorbable high frequency noise. In other examples, some conventional engine noise reduction systems use suction devices. Alternatively, suction devices have been used to reduce aerodynamic drag.
Various techniques have been developed to reduce airframe noise, defined as objectionable audible noises during departure and approach conditions from an aircraft and induced by airflow, not related to the engine during operation. Airframe noise can reach or exceed engine noise levels during aircraft landing. Conventional techniques typically address airframe noise by thickening the shear layer adjacent the end of the aircraft body by positioning protuberances adjacent the end of the body. Unfortunately, the devices attached to an aircraft may introduce new noise sources even while reducing some airflow-related noise.
Known techniques successfully reduce noise levels, at least to some degree. However, further reductions are always desirable. Furthermore, the conventional techniques impact performance by one or more of adding weight to the aircraft, reducing engine performance, reducing aerodynamic performance, increasing fuel consumption, reducing range, and/or increasing engine complexity in ways that can compromise engine performance and reliability, increasing the possibility of breakdown and increasing cost.
Noise abatement flight procedures are constantly evolving with advances in technology, improved aircraft design, and implementation of airspace management procedures. Many efforts to address aircraft noise have been targeted to reduction of noise at the source. Aircraft are required to meet government noise certification standards. Compliance with these standards must be considered in the design of new aircraft.